Carts, tables, casters directly mounted to the frame, and other means are used to support/transport computers and other delicate electronic equipment. The performance of such equipment can be degraded by vibration during usage and shock loads experienced during transport on such carts, and both can produce various degrees of damage to the equipment.
Previous attempts to develop a shock-isolating caster have met with limited success. This is due, in part, to the rigorous demands placed on such casters, since each may experience static loads in excess of 1,000 pounds each and shock loads of 100 times the static weight load. Further, a number of these attempts employing elastomer to isolate rely upon the elastomer to dissipate energy in shear. U.S. Pat. No. 3,072,169 issued to Hastings, Jr. and U.S. Pat. No. 3,194,293 issued to Kindley are two such caster designs which rely upon elastomeric shear to isolate shock loads. The shear action produces hysteresis damping which dissipates the vibration or shock load energy as heat. In both of these caster configurations, the engineer must trade off competing design criteria. For optimal stability, the elastomer should be as hard as possible. However, in order to provide the desired isolation characteristics, a low modulus (soft), highly damped elastomer must be used.
In the caster designs of the present invention, surface-effect damping, which has both hysteresis and friction components, is used. The addition of friction damping to hysteresis damping affords much higher levels of energy dissipation while permitting the use of lower modulus elastomers. This unlinks, to some extent, the competing design characteristics of stability and isolation and allows the designer greater leeway in selecting an elastomer that will satisfy both requirements.
The present invention is a shock-isolating caster comprising a ground-engaging wheel element; a mounting member for securement to a base assembly to be isolated from shock and vibration loads; intermediate structure positioned between said ground-engaging wheel element and said mounting member; a surface-effect elastomeric damper engaging at least a surface portion of one of said wheel element and said intermediate means, said surface-effect elastomeric damper producing both frictional and hysteresis damping forces upon said surface portion which it engages to dampen transmission of vibration and shock loads to said base assembly from said wheel element.
In a first embodiment, the intermediate means comprises a bracket attached to the mounting member, an axle supported by the bracket, which axle extends through a friction-reducing bearing that receives the wheel element. In this embodiment, the elastomeric damper reacts between an outer wheel element and an inner wheel element to damp transmission of shock and vibration loads from the wheel to the base assembly to which the caster is attached.
In a second and third embodiment, the wheel element is secured to one end of an elongated pivot arm and the elastomeric damper engages the pivot arm (the intermediate means) and not the wheel. In the second embodiment, the pivot arm is a length of spring steel and the damper comprises a saddle support with elastomeric members bonded to inwardly directed, opposing lateral surfaces. The elastomeric members are engaged by lateral faces of the spring arm which produces hysteresis and friction damping of relative movement between wheel and base assembly. The third embodiment positions the damper between the saddle support bracket and a C-shaped mounting arm with an upper rib on the arm, which arm is pivotally mounted to the support bracket, a damper positioned between the ends of the bracket and the arm which are distal from pivot, the rib on the arm being engaged in a recess in the damper to provide friction and hysteresis damping.
Various other features, advantages and characteristics will become apparent after a reading of the following Detailed Description of the Preferred Embodiments.